Fabrication of low cost, high volume, tubular metal consumer products such as bicycle frames, household furniture, automative exhaust systems, lawn and garden equipment and numerous similar structural elements of other such products depends to a significant degree upon the use of low cost methods of joining hollow metal tubing by means of arc welding, electron beam welding, laser welding and brazing or soldering.
The most economic design of these products requires the use of joining methods having a high degree of reliability. The juncture of the tubular elements must be free of sharp notches and corners, voids and foreign inclusions, since operating stresses will often concentrate in such areas of the structure and result in premature fatigue failure. Successful joining of such tubular elements requires that careful consideration be given to the fit of abutting surfaces prior to fusion or welding in order to consistently achieve adequate corner fillets and a smooth transition of the joined surfaces. As an example, laboratory experiments have indicated that consistently satisfactory brazed joints require that all elements of abutting surfaces must be within 0.002 to 0.005 of touching each other, depending, to some degree, upon the filler alloy used.
Historically, numerous methods have been employed to achieve the desired fit of such abutting tubular surfaces. See, for example, U.S. Pat. Nos. 2,746,125; 3,073,195; 3,180,196; and 3,881,385, each of which discloses a system for shaping the end of a tube so that it may be joined to another tubular element.
The most economic of the prior art tube notching methods employs linear shear of the tubing through the use of a suitable punch and die. A common practice in such existing methods is to enclose the tubing about its external peripheral area in a suitable die block having a means for holding and guiding a punch of suitable contour and at a desired angle. The punch is then forced through the hollow tube with sufficient force to shear away the material necessary to produce the desired contour at its intended abutting end. Because the tube wall initially contacted by the punch is not supported internally this method produces substantial deformation at the entry side of the tubing and requires secondary reformation and finishing.
Another popular method involves the similar use of a punch and die wherein the tubing is positioned in the die block so that the punch may enter the open end of the tubing and shear away one half of the desired shape. The tubing is then withdrawn, turned about its axis through 180.degree. and reinserted in another suitable die so that the punch can enter the previously notched end and produce the remaining one half of the desired contour. This method requires careful positioning of the tubing and control of the registry of the previously made one half of the notched contour. It often results in secondary finishing and is relatively time consuming.
Other methods used for contour notching the ends of tubular elements may employ a band saw or a rotatable cutter of the type used for end milling. Such a rotating cutter, having a cutting diameter appropriate to the contour desired, is forced through the tube at a suitable angle, while the tube is held in a suitable vise or holding fixture. While this method generally results in a precision of fit of the contoured end, it is time consuming and requires a substantial capital investment to provide sufficient equipment to support large volume production.